This invention relates to an image sensor comprising an insulating substrate carrying an array of discrete photosensitive elements each having a given surface area for detecting light incident on the photosensitive element and an array of lens elements provided over the photosensitive array so that each lens element is associated with a respective photosensitive element for concentrating light incident on the lens element onto the associated photosensitive element.
EP-A-154962 describes such an image sensor in which the photosensitive elements are formed on top of a light shielding layer provided on a light transmissive substrate and the array of lens elements is fabricated either on top of the photosensitive array or on the other side of the light transmissive substrate by one of several different methods, for example using photosensitive resist or by ion implantation into a light transmissive substrate so as locally to alter the refractive index of the substrate. An object to be imaged is placed in front of the lens array and light transmissive regions in the light shielding layer allow light to pass through the substrate to be incident on the object, for example a document, to be imaged. Light reflected by the object is incident on the lens elements and each lens element serves to concentrate, generally focus, the incident light onto the associated photosensitive element.
The use of such lens elements enables a short focal length to be achieved and enables the pitch of the photosensitive element array to be reduced allowing the optical path length to become equivalent to that of a comparable intimate contact image sensor. Such an arrangement enables one-to-one correspondence images to be formed without the need for intimate contact between the object being imaged and the photosensitive array whilst avoiding the problems of possible electrostatic and mechanical damage inherent in an intimate contact image sensor. As described in EP-A-154962, the photosensitive elements used are small and so have a small surface area in comparison to the size of the imaging area, that is the area over which each lens element collects light, because of the focusing effect provided by the lens element. The use of such small photosensitive elements restricts the spread of angles of incidence (the acceptance angle) over which the image sensor receives light which should assist in reducing the possibility of light from one imaging area being focused onto two or more adjacent photosensitive elements rather than simply onto the photosensitive element associated with the lens element which is intended to receive light from that imaging area and thus should improve the resolution and thus the sharpness of the obtained image. Also, the use of such small photosensitive elements should assist in reducing leakage currents. This however requires accurate control over the formation of the photosensitive elements and because, inter alia, of the inherent thickness of the photosensitive elements it can be difficult to form extremely small photosensitive elements and there will in practice be a limit to the degree to which the size of the photosensitive elements can be reduced without detrimentally effecting the reproducibility of the manufacturing process and thus of the characteristics of the image sensor.